直接上程式碼：

    /*
     * 獲取當月有多少天
     * @param date $unix
     * return int
     */
    public static function getDaysByMonth($unix){
        $month = date('m', $unix);
        $year = date('Y', $unix);
        $nextMonth = (($month+1)>12) ? 1 : ($month+1);
        $year      = ($nextMonth>12) ? ($year+1) : $year;
        $days   = date('d',mktime(0,0,0,$nextMonth,0,$year));
        return $days;
    }